In a known metering apparatus of this kind for lapping medium, the desired amount of liquid is withdrawn from a stream of liquid flowing in laminar form in a free jet by the collecting pipe having its collecting opening immersed in the stream of liquid to a predetermined depth. Since, however, this immersion is effected from the outer boundary of the stream of liquid and this outer boundary may shift during such withdrawal as a result of possible variations in the state of flow of the liquid, this method does not enable the adjusted metering to be maintained.
Metering devices are also known in which the metering is effected via a throttling element which is installed in the feed line and which must be adjusted at any given time to the quantity to be metered and to the consistency of the lapping medium. The lapping powder, however, produces an erosion at the narrow parts of the throttling element, which alters the adjustment thereof and, moreover, renders exchange of the throttling element necessary after a certain time. Furthermore, throttling elements have a tendency to become blocked, depending upon the size of the grains of the lapping powder employed, which necessitates a costly device for keeping the bores of the throttling elements clear.
The problem underlying the invention is to provide a metering apparatus of the kind mentioned at the beginning which renders possible accurate and reliable, as well as trouble-free, metering of liquids containing granular solids, in particular lapping liquid.
According to the invention, this is achieved in that the stream of liquid issuing from the feed pipe is conveyed with a wide area against a substantially vertical wall, to run down thereon in the form of a thin film of uniform thickness, the collecting pipe is arranged essentially on that side of the wall which is remote from the liquid film, the wall has a through orifice in its lower zone for the free end extending obliquely upwards of the collecting pipe, and the collecting opening has a cross-section which widens with progressive movement of the collecting pipe into the through orifice.
Since the liquid is conveyed with a wide area on to the vertical wall, a uniform distribution of the liquid in the form of a thin liquid film takes place at this wall. The thickness of this liquid film is scarcely subject to variation. The collecting opening of the collecting pipe "cuts out" of the liquid film a strip whose width is dependent upon that cross-section of the collecting opening which is located at any given time in the region of the wall. In this way, a very simple and trouble-free metering apparatus is created which enables accurate metering of the liquid, in particular a lapping liquid.
There has been found to be particularly advantageous a metering apparatus wherein the wall is formed by a cylindrical casing with a vertical axis into which the coaxially arranged feed pipe extends from above, the lower end of the feed pipe being arranged at a distance from a horizontal coaxial impingement plate by which the liquid is conveyed radially outwards against the inner face of the cylindrical casing. By means of the impingement plate, the stream of liquid is converted into an umbrella-shaped curtain of liquid and deflected radially outwards. The curtain of liquid striking against the inner face of the cylindrical casing forms thereon a thin liquid film of uniform thickness.